Lower phosphate fertilizer prices have generated some interest in building soil phosphorus (P) levels and questions concerning this practice. Why would someone want to build soil phosphorus levels?
Soil phosphorus is relatively stable and will not be easily lost like nitrogen, which is mobile in the soil as nitrate-N. In most soils, the phosphate ion will move into more stable forms and be released as the plant requires more P. Therefore, if current P fertilizer prices are lower than expected future prices, producers who own the land may wish to “store” the less expensive P on their fields, especially those with lower soil P tests.
Building soil test P levels from low to high will increase yield, even if no pre-plant P fertilizer is applied to those fields. In long-term studies at South Dakota State University (SDSU) it was found that low soil test P levels yielded as well as very high test P levels when the appropriate P rates were applied. Therefore, if one builds soil test P levels from medium to very high, there is little need to apply additional P, except for adding a small amount with a starter. This practice of P management allows for reducing P rates in those years when P fertilizer prices are higher than average.
How much P fertilizer does one need to increase soil test P? A lot is the short answer! Research at SDSU and many other Midwestern universities have shown an average of 18 to 20 pounds of applied phosphate fertilizer (P2O5) per acre is needed to raise soil test P one ppm. The 18- to 20-pound rate is over and above any P applied to replace P removed with the harvested crop.
Newly revised P removal rates are 0.27 and 0.61 pounds of phosphate (P2O5) per bushel of corn and soybean, respectively. For example, with a 180-bushel-per-acre corn crop, phosphate removal is about 49 pounds of P2O5/ac, and for a 60-bushel-per-acre soybean crop, removal is about 37 pounds of P2O5 /ac. Therefore applying, for example, 120 pounds of actual P2O5 (230 pounds of 11-52-0) for each year in a corn-soybean rotation would raise soil tests about 6 ppm [(120x2) – (49+37) = 114/20 = 6].
It can be a slow and fairly expensive process to build soil P tests. Are there some soils that will not build or build very slowly? Yes — unfortunately, we don’t have these identified, although we believe there are not many in the state. The build rate of 18 to 20 pounds of fertilizer P2O5 to raise soil test by 1 ppm is an average of many soils, although most were medium to fine textures. Coarse soils with less clay tend to take less fertilizer to raise soil test, whereas heavy clays, tend to take more P to raise soil tests.
In addition, higher pH soils (>7.5) especially with free calcium carbonate tend to take more fertilizer P to raise soil test P. In saying this, we have had studies on high clay soils with free calcium carbonate that have similar build rates as above. For some soils, we just don’t know the phosphorus build rate that is needed.
If you have applied ‘extra’ P fertilizer above what the crop removed for several years and the field or areas within the field have not changed in soil test, these soils are likely ‘fixing’ added P and will not release this P easily to the plant. Crops growing on these soils typically respond to band applied P better than broadcast P.
Annual soil P tests should be taken to monitor trends and adjust applications accordingly. Don’t use just one year’s soil test to make soil test P adjustments, but at least use 2 to 3 test years to determine trends.
The bottom-line for increased soil P test levels is that it can be a slow and expensive process. Building soil P levels should only be used if fields are owned and then only if current fertilizer P prices are significantly lower than typical prices.